The present application relates to the reduction of engine noise and, in particular, to an acoustic attenuator for an air compressor of an engine booster. In the case of a turbocharger engine during transient maneuvers, broadband aero-acoustic noises can be generated by the compressor dynamics. The acoustic pressure waves can propagate upstream of the compressor against the flow of air and be radiated via the various components forming a low pressure air supply for the turbocharger. In addition, when the pressure produced by the turbocharger exceeds a predetermined value in tip-out maneuvers, it is usual for a compressor bypass valve to open. The opening of this valve can generate broadband acoustic pressure waves in a backflow direction and an audible ‘whoosh’ noise that is radiated via the various components forming the low pressure air supply for the turbocharger.
U.S. Pat. No. 6,752,240 provides a reactive noise reducing device connected to an inlet of an air compressor of a supercharger for an engine. Such a device has the disadvantages that it is of relatively large size due to the need to provide a number of different chambers if different frequencies are to be silenced. This is because a specific chamber dimension is required to reduce specific frequency ranges. Such an arrangement is very inflexible in terms of operation and has to be designed to fit a specific supercharger installation. That is to say, if the same supercharger is used on a different engine requiring a different air inlet system design this type of noise reducing device may not provide adequate noise attenuation due to the different frequency ranges that may be produced.
Some embodiments described herein provide an attenuator for an engine booster that overcomes the problems referred to above. According to a first aspect, there is provided an acoustic attenuator for an engine booster comprising an attenuator body defining an air flow passage through which low pressure air flows to an air compressor of the booster and an attenuator chamber containing acoustic pressure wave absorbing material operatively connected to the air flow passage via a number of transfer ports wherein the acoustic attenuator is located close to an inlet port of the air compressor.
One end of the attenuator body is adapted for connection to an inlet port of the air compressor. The body may be adapted for direct connection to the inlet port of the air compressor or may be adapted for indirect connection by being connected via a short spacer component such as a tube. The attenuator chamber may extend around only a portion of the attenuator body. The portion may be an upper portion, in a vertical direction relative to a surface on which a wheel of the vehicle rests. Each of the transfer ports may be formed by an elongate aperture aligned with the general flow path of air through the air flow passage.
The acoustic pressure wave absorbing material may be one of a fibrous mat, foam and a combination of foam and a fibrous mat. The attenuator chamber may house at least two acoustic pressure wave absorbing materials having differing frequency absorbing properties. The attenuator chamber may be formed by a separate attenuator housing that fits in an aperture in the attenuator body. The attenuator housing may comprise first and second end walls, first and second side walls and a floor in which a number of apertures defining the transfer ports are formed and a cover securable to the attenuator so as to form a lid for the attenuator housing.
According to a second aspect, there is provided a low pressure air supply system for an engine having a booster, the system comprising a low pressure air inlet through which atmospheric air is drawn into the system, an air filter for filtering the air drawn in via the low pressure air inlet and a low pressure air conduit connecting the air filter to an inlet end of an acoustic attenuator constructed in accordance with said first aspect wherein the acoustic attenuator is located close to an inlet port of an air compressor of the booster.
The acoustic attenuator has an outlet end adapted for connection to an inlet port of an air compressor of the booster. The attenuator body may be adapted for direct connection to the inlet port of the air compressor or may be adapted for indirect connection by being connected via a short spacer component such as a tube.
According to a third aspect, there is provided a motor vehicle having an engine, a booster connected to the engine so as to provide a boosted air supply to the engine and a low pressure air supply system constructed in accordance with said second aspect connected to the booster so as to provide a supply of low pressure air to the air compressor of the booster.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.